In present day broadband cable communications, Quadrature Amplitude Modulation (QAM) is typically employed to carry signals over multiple QAM channels that are distributed within a set frequency range. For example, multiple single carrier QAM channels approximately 6-8 MHz wide may be distributed over an allotted frequency range to carry different cable signals. To increase data capacity, future cable systems may employ orthogonal frequency division modulation (OFDM), which may entail channels having bandwidth in the neighborhood of 200 MHz. As OFDM technology is deployed in cable communications, it is envisioned that cable communications bands may need to support the ability to transmit both OFDM and QAM channels over a given physical cable. Thus, when wideband OFDM channels are deployed in an existing network that employs QAM, it is necessary to make allowance for existing single-carrier (6 or 8 MHz) QAM channels, e.g., legacy QAM channels, which may still be used to carry cable communications.
One approach that has been proposed to promote coexistence of QAM and OFDM channels is to employ wideband OFDM channels in which certain OFDM carriers are nulled in the spectral regions of legacy QAM channels. However, even if carrier nulling of the OFDM spectrum of the carriers on either side of a legacy QAM channel is implemented, significant interference to the legacy channel from OFDM signals may occur. It has been proposed to perform windowing of OFDM symbols to reduce this interference, which may be conducted at the transmitter end. A small amount (1%) of windowing may be effective in stopping the OFDM power from spreading into the legacy QAM channel region. This allows the OFDM carriers that are not nulled to be placed very close (within 1 MHz) to the edge of the legacy QAM channel without causing any interference to the legacy co-channel. In the above manner, QAM legacy channels may continue to carry cable signals without significant interference from new OFDM channels.
A converse of the problem of interference created in QAM channels by OFDM carriers is the potential interference created by legacy QAM channels on adjacent OFDM carriers. In particular, when the OFDM fast Fourier transform (FFT) is taken of a composite channel signal including OFDM and QAM channels, the co-channel interference (CCI) caused by legacy QAM channels exhibits a severe impact on many OFDM carriers that reside on either side of a QAM channel. This is because OFDM FFT entails applying a rectangular window function to the signal, which window function results in a significant spectral spread of legacy channel power in the frequency dimension. This interference may result in a loss of data carrying capacity that is undesirable or unacceptable, especially in cases in which more than one legacy QAM channel may be present among OFDM channels.
An additional source of interference for OFDM cable channels is that created by wireless devices, such as smartphones, cellular telephones and other devices that may be brought into proximity with a cable modem or other cable device receiving the OFDM signal. In some instances, a wireless device may be transmitting to a base station within a cable bandwidth (800-1000 MHz), and as a result may create severe interference to cable channels.
Accordingly, there may be a need for improved techniques and apparatus to solve these and other problems.